Integrated circuits consist of functional devices, interconnects and isolators formed on or within the surface of a support substrate. Silicon wafers are the most common substrates. The components are formed in several sequential process steps.
In the manufacture of integrated cirucits or chips, a photoresist coated silicon wafer is exposed to radiant flux which interacts with the exposed portions of the photoresist material.
The interaction alters the molecular properties of the exposed photoresist. The photoresist materials fall into two major categories that are affected by exposure in opposite ways. One group, generalized as positive working, becomes soluble in developer only in those areas exposed to the radiations. Unexposed areas are not removed by the development and thus remain on the substrate.
The second group, negative working, becomes immune to developer action in the exposed areas. unexposed areas are removed by development.
Circuit patterns are placed on the wafer by exposure of the photoresist coating through a mask, containing transparent and opaque lines and areas, which defines the circuit pattersn or other elements.
The developed photoresist layer forms a process mask or shield that defines the areas to be affected by the subsequent process step.
Each mask comprises a plurality of the same patterns disposed in rows and columns so that after being fully processed the wafers are cut along rows and columns to provide a number of identical integrated circuits of chips.
The photoresist materials used for most integrated circuit fabrication are fundamentally responsive for exposure to radiation in a spectral region which includes the middle and near ultraviolet and visible blue light.
A convenient source of this radiation is the high pressure mercury arc lamp. The flux output of a mercury arc source is not smoothly distributed across the spectrum of interest but is characterized by a series of intense peaks or relatively narrow bandwidths superimposed on a continuum.
The peaks are inherent in their spectral placement but their relative intensities are a function of variabilities of lamp operation and construction.
The photoresist materials are not uniformally responsive to the available spectrum range from the mercury arc source.
In general, a resist is most responsive (and possibly optimized) to a narrow spectral region containing one of the spectral peaks.
One method of exposure of a photoresist coated wafer to circuit patterns of a mask is disclosed in U.S. Pat. No. 4,011,011 entitled Optical Projection Apparatus issued Mar. 8, 1977, and having the same assignee as a present application. In the patent, a carriage, holding the mask and wafer in aligned relationship, is scanned past a selected area illuminated by a light source.
The total energy incident on a given point of the photoresist can be controlled by the speed of the scanning motion.
If the illumination source were constant spectrally, and in intensity, it would be a simple matter to determine the optimum exposure time for a particular photoresist, and to set the speed of scan of a carriage, as disclosed in the above referenced patent.
Any change in emission level is monitored by a detection system that derives a control signal. This signal is used to change the scanning motion speed and thus correct the exposure. This system is embodied in the above referenced patent.
In this aforementioned system a compromise has been made in the spectral matching of the detector characteristics to the mercury lamp and photoresist properties.
A problem can be encountered if photoresist is used that has a different spectral response from that being monitored by the emission level detector, and if the lamp emissions change in a non uniform manner spectrally.
The present invention relates to an apparatus for providing a control signal which changes in accordance with variations in the output of a light source when measuring at one or more points in the emitted spectrum.